Air cushion dredge for use in ice-covered waters

ABSTRACT

Apparatus for arctic dredging and marine pipelaying during icefree and iced conditions is described. For dredging, a dredging facility is carried on a buoyant platform which is adapted for support on a cushion of air. The dredging facility includes a dredging head which is operable, when engaged with the bed of a body of water to be dredged, for removing material from the bed. A dredgings conductor tube is connectible between the dredging head and the platform for conducting dredged material from the head to the platform. The dredging equipment also includes means carried by the platform for forming a channel through a layer of ice below the platform for passage of the conductor tube through the ice when the dredging apparatus is operated during iced conditions.

1 ited States 1 aten 1191 [11] 3, Blankenship Dec. 9, 1975 AIR CUSHIONDREDGE FOR USE IN 3,632,172 1/1972 Robinson et 81.... 299/27 XICECOVERED WATERS 3,688,850 9/1972 Knorr 180/127 X 3,693,729 9/1972Blurton et al.... 61/46.5 X lnventorl Blankenship, Houston, 3,763,580/1973 Kuntz, Jr. ..L 37/65 x Tex. [73] Assignee: Global Marine, Inc.,Los Angeles, Primary ne -Clifford D. Crowder Calif. Attorney, Agent, orFirm-Christie, Parker & Hale 22 Filed: Apr. 22, 1974 57 AB T T [21]Appl. No.: 462,749 1 f d d F d l pparatus or arctic re ging an marinepipe aymg Related Apphcatlon Dam during ice-free and iced conditions isdescribed. For [62] Division of Ser. No. 276,110, July 28, 1972, Pat.No. dredging, n dredging facility is carried on a buoyant 313225556platform which is adapted for support on a cushion of air. The dredgingfacility includes a dredging head U-S- Cl. is operable when engaged thebed of a 372/195; 61/724; 114/42; 180/127; 299/ body of water to bedredged, for removing material [5 Clo i from the A dredgings conductortube is connect- Fleld of Search 114/40, ible between the dredging headand theplatform for 114/411 67 67 R; 61/723 465; conducting dredgedmaterial from the head to the 299/24, 25, 26, 27, 28, 18; 37/54, 53, 64,platform. The dredging equipment also includes 65, 66, 67, means carriedby the platform for forming a channel through a layer of ice below theplatform for passage References Clted of the conductor tube through theice when the dredg- UNITED STATES PATENTS ing apparatus is operatedduring iced conditions.

3,362,500 H1968 Bliss 180/128 3,473,624 10/1969 Mezhlumov et al. 180/1276 Clams 6 Drawmg figures US. Patent Dec. 9, 1975 Sheet 1 of4 3,924,896

r l ll II I. ll l l l MM w US. Patent Dec.9, 1975 Sheet2of4 3,924,896

sheet 3 OH 3,924,896

US. Patent Dec. 9, 1975 US. Patent Dec. 9, 1975 Sheet4 0f4 3,924,896

F/EE

AIR CUSHION DREDGE FOR USE IN ICE-COVERED WATERS CROSS REFERENCE TORELATED APPLICATIONS This is a division of application Ser. No. 276,110,filed July 28, I972, 3,822,558. US. Pat. No. 3,822,558.

FIELD OF THE INVENTION This invention pertains to marine arctic dredgingand pipelaying. More specifically, this invention pertains to method andapparatus for performing dredging and pipelaying operations, in waterssusceptible to being covered by a layer of ice, during both iced andice-free conditions.

BACKGROUND OF THE INVENTION Substantial reserves of oil and gas areeither known to exist or are suspected to exist in areas lying offshorefrom northern Alaska and Canada. The precise seaward extent of thesereserves from the adjacent shore line has not yet been defined withprecision. It is know, however, that many of the established reserveslie under water which is covered by ice for many months of the year andwhich, during any given year, may be covered yearround by ice.

Oil or gas may be produced from a suitable well only when, and to theextent that the oil or gas can be removed directly from the wellhead toa suitable storage facility or pipeline. In the case of offshore wells,the provision of local storage facilities is not possible, particularlyin the arctic. Accordingly, the successful and economic exploitation ofthe known and suspected oil and gas reserves in the arctic is dependentupon the provision of pipelines from the several wells across the oceanfloor to centralized land-based storage and shipment facilities. Thesefacilities may be adjacent a harbor which is ice-free for a sufficientperiod during every year to enable tankers and the like to collect andremove, within a few months time, all of the oil and gas producedannually from the wells in the area. On the other hand, these facilitiesmay be a central collection point for a large diameter gas or oilpipeline by which the production from the adjacent oil and gas fieldsmay be transported, on a year-round basis, to remote locations offurther use or processing.

The economic feasibility and desirability of constructing oil or gaspipelines across the land of northern Alaska and Canada are topics whichhave not yet been resolved, due primarily to the fact that suchpipelines would have to be constructed across the arctic tundra. Thetundra presents delicate and complex ecological problems. In the extremenorth, the surface vegetation is delicate and, if destroyed, only slowlyreestablishes itself. Surface vegetation is particularly vital to thepreservation of the arctic terrain due to the presence of permafrostunder the surface vegetation. Surface vegetation provides a naturalinsulative barrier to complete thawing of the permafrost during thearctic summer. The destruction of the surface vegetation leads to rapidand substantial degradation of the underlying permafrost layer duringthe arctic summer, and such degradation, in turn, produces markedsubsidence of the land level and often disastrous interference with thenatural water courses in the area. For example, the roadways acrosswhich military vehicles moved in Alaska during the arctic summers ofWorld War II are now visible as 2 trenches 8 feet or more in depth and40 feet or more in width; these trenches are the direct result of thedestruction of the surface vegetation by such vehicles.

These ecological considerations, therefore, argue strongly against theconstruction'by conventional techniques of oil and gas pipelines acrossthe tundra of North America to serve the substantial oil and gasreserves known to exist in northern Alaska and Canada. Similarly, theseconditions are :relied upon by ecologists and others to urge that anyacross-land pipelines in the arctic be fabricated by unconventional, andtherefore costly, techniques.

An alternative solution to the problem of removing oil from the northernAlaskan and Canadian petroleum reserves to more southerly points ofconsumption and refinement is the use of marine pipelines from the oiland gas fields to points sufficiently far south that the ecologicalproblems associated with across-land arctic pipelines are notencountered. Such long distance marine pipelines like the smaller localpipelines necessary to interconnect offshore wells to central collectionand storage facilities, would have to be laid under waters which arecovered by ice for many months of the year of which, in a given year,may be covered year-round.

The marine pipelaying procedures and devices described in the prior artmay be used only to lay pipeline under waters which are ice-free.Therefore, reliance upon existing procedures and equipment to constructlong distance marine pipelines in the arctic is feasible during only afew months of the year, and may not be possible at all in some areaswhere ice may exist on a year-round basis. The construction of longdistance marine pipelines in the arctic by existing techniques andequipment would be prohibitively expensive due to the short constructionseason afforded in these areas and the need to move pipelaying equipmentinto and out of construction areas during the ice-free period.Dependence upon existing marine pipelaying proce dures and equipmentmeans that if long distance marine pipelines are to be used to provide asolution to the ecological problems reviewed. above, such pipelineswould require several years to construct and would be far more costlyper mile than any other pipeline now known.

In the view of the foregoing, it is apparent that great need exists forprocedures and equipment which will enable the construction on asubstantially year-round basis and at reasonable cost of marinepipelines through and under waters which are susceptible to coverage byice for substantial periods of the year.

SUMMARY OF THE INVENTION This invention provides novel, economic,efficient and practical procedures and equipment which may be used toadvantage substantially on a year-round basis in the arctic for theconstruction of submarine pipelines. The procedures and equipmentcontemplated and comprehended by this invention may be used duringperiods when the waters over the path of the pipeline are ice-free orare covered by ice. This invention, therefore, makes possible theeconomic construction on an essentially year-round basis of marinepipelines which may be used as substitutes for or alternates tocross-land pipelines.

Generally speaking, this invention provides a method for laying apipeline and the like across the bed of a body of water covered by alayer of ice. The method includes the steps of dredging a trench in thebed along the path of the pipeline ,from a dredging operations stationsupported on the ice on a cushion of air, and laying the pipeline in thetrench from a pipelaying operations station supported on the ice on acushion of air. Preferably, the dredging and pipelaying operationsstations are buoyant so that dredging and pipelaying operations may beperformed during periods when the water is icefree as well as duringperiods when the water is covered by a layer of ice.

This invention also provides a method for dredging in water susceptibleto being covered by a layer of ice. A dredging facility is located on abuoyant platform adapted to be supported on a cushion of air. Theplatform is floated in the water and dredging operations are performedfrom the platform during periods when the water is not covered by icesufficient to restrict floating movement of the platform. During periodswhen the water is covered by ice sufficient to restrict floatingmovement of the platform, the platform is supported on a cushion of airand dredging operations are performed from the platform through the ice.Preferably, the dredging facility includes a dredging head which isoperative to remove material from the bed of the water body, and adredgings conductor pipe which is connected during dredging operationsbetween the dredging head and the platform. Means are carried by theplatform for forming through an ice layer below the platform a channelthrough which the dredgings conductor pipe may be passed.

DESCRIPTION OF THE DRAWINGS The above-mentioned and other features ofthis invention are more fully set forth in the following description ofcertain presently preferred embodiments of the invention, whichdescription is presented with reference to the accompanying drawings,wherein:

FIG. 1 is a cross-sectional elevation view of a dredging and pipelayingfacility according to this invention;

FIG. 2 is a top plan view of the dredging and pipelaying facility shownin FIG. 1;

FIG. 3 is a cross-sectional elevation view of the mechanism provided inthe air cushion platform shown in FIG. 1 for forming a clearance channelthrough the ice layer for the dredgings conductor pipe and the pipeline;

FIG. 4 is a simplified top plan view of a dredging facility according tothis invention in use during iced conditions;

FIG. 5 is an enlarged fragmentary cross-sectional elevation view of aportion of the structure shown in FIG. 4; and

FIG. 6 is a cross-sectional elevation view of another dredging apparatusaccording to this invention.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS An air cushion vehicle 10 isdisposed over a sheet of ice 11 formed on the surface 9 of a body ofwater 12 which has a bottom or bed 13. The vehicle is comprised of abuoyant platform-like hull 14 having a substantially flat deck 15 and asubstantially flat bottom 16. A plenum'structure 17 for the distributionof pressurized air extends circumferentially of hull 14 adjacent itsgunwale 18. A flexible skirt assembly 19 is connected to the lowerportions of the plenum structure and is arranged to depend below thebottom 16 of the hull to enclose a space 20 between the hull and theupper surfaces of ice sheet 11 when vehicle 10 is operated in its aremounted to deck 15 adjacent the aft end thereof,

and are operated by a suitable prime mover 23 (see FIG. 2) which may bean electric motor or a diesel engine, for example.

A forward portion of vehicle 10 defines a dredging station 25 withinwhich a dredging facility 26 is mounted to hull 14. The dredgingfacility includes a dredging head 27 which is carried by the lower endof a dredgings conductor tube 28 which extends downwardly through hull14 via a well 29 from a pump 30. The upper end of tube 28 is connectedto the suction port of pump 30. A discharge conduit 31 is connected tothe discharge port of the pump and extends to ad jacent the gunwale ofthe hull where it is supported on a boom 32 (see FIG. 2). Boom 32extends laterally of the side of vehicle 10 from a suitable king post 33or the like. The boom is provided for supporting conduit 31 above icesurface 11 for a desired distance laterally away from vehicle 10 so thatdredgings gathered from bed formation 13 by dredging head 27 andconducted to pump 30 by tube 28 may be discharged via conduit 31 ontothe ice surface a suitable distance laterally from vehicle 10. Dredginghead 27 preferably is of the suction type which operates in response tothe operation of pump 30 to gather the material of bed formation 13 fromthe bed in the vicinity of the dredging head. Accordingly, as vehicle 10moves across ice 11 along a desired path, a trench 35 is formed bydredging head 27 in bed formation 13 below vehicle 10.

In order that dredging facility 26 may be operated to form trench 35during air cushion mode operation of vehicle 10, a suitable seal 36cooperates between the structure of hull 14 and the dredgings conductortube within well 29 so that super-atmospheric pressure may effectivelybe generated and maintained within space 20 by operation of blowers 22.

The dredging facility illustrated in FIGS. 1 and 2 and described aboveis merely exemplary of a dredging facility which may be provided in anyapparatus accord ing to this invention for the performance of theprocedures contemplated by this invention.

As shown most clearly in FIG. 2, the area of vehicle 15 aft of dredgingstation 25 is designated as a pipelaying station 38 and is the locationin vehicle 10 where a pipelaying facility 39 is located. The pipelayingfacility includes a pipe welding assembly 40, preferably located alongthe centerline 48 of hull l4 forwardly of a pipe holding and tensioningassembly 41. The pipe welding and pipe holding assemblies may be ofconventional configuration and arrangement. Pipe welding station 40 isprovided for welding the rear end of a pipe length 42, disposed abovedeck 15 on suitable supports 43 (see FIG. 1), to the forward end of aplurality of pipe lengths which have been welded together to define acontinuum 44 of pipe. The pipe continuum extends from welding assembly40 through pipe holding and tensionin g assembly 41 aft of vehicle 10and into trench 35, as shown in FIG. 1. A portion of the pipelinecontinuum 44 disposed in trench 35 and supported by formation 13 at thebottom of the trench constitutes a portion of a submarine pipeline 45being laid under body of water 12 by operations carried out on and fromthe platform defined by vehicle 10. Pipe storage areas 46 are providedlaterally from pipe welding assembly 40 as additional components ofpipelaying facility 39.

As shown in FIG. 1, pipelaying vehicle 11) is arranged for passage ofpipe continuum 44 through hull 14 and into water 12 within the area ofair cushion support of hull 14 when operated in its air cushion mode.Accordingly, a passage 47 is provided through hull 14 along thecenterline of the hull. Passage 47 slopes downwardly and rearwardly fromits upper end at deck 15 to its lower end through bottom 16 of the hull.A suitable seal 49 cooperates between the walls of the passage and thepipe continuum within the length of the passage for preventing undueleakage of pressurized air through the passage from space 20 so that aregion of air at superatmospheric pressure may be generated below hull14 in response to the operation of blowers 22.

It is apparent that when vehicle 10 is operated over ice 11, somemechanism must be provided for forming openings through ice 11 for thepassage of dredgings conducting tube 28 and pipe continuum 44 intocontact with bed formation 13. Accordingly, pipe-laying vehicle 10includes an ice cutting assembly 51) for forming an open channel 51through ice sheet 11 of sufficient width that the dredgings conductortube and the pipe continuum with its associated supportive stingerassembly 52 may be passed through the ice sheet. As shown best in FIG.3, ice cutting assembly 50 includes a centerline ice cutting mechanism54 and a pair of side ice cutting mechanisms 55 disposed on oppositesides of and equidistantly from centerline mechanism 54. Each ofmechanisms 54 and 55 is provided in the form of an enlarged chain sawhaving an elongate blade 56 (see FIG. 1) the circumference of whichdefines a guide-way for an encircling tooth-carrying chain 57. Eachchain carries a plurality of teeth 58 and 58 adapted upon operation ofthe associated mechanism for cutting ice with which the teeth may bebrought into contact. The blades of ice cutting mechanisms 54 and 55 areadapted to be movable between an operative position in which they dependfrom the bottom 16 of hull 14 through ice layer 11 and a retractedposition in which the blades are disposed in respective elongatedrecesses 60 formed in hull 14 to open downwardly from the hull intospace 20.

Recesses 60 are defined within the interior of corresponding elongatehousings 61 which extend upwardly within hull 14 from the bottom of thehull, as shown in FIG. 3. Housings 61 are parallel to each other andextend fore-and-aft within hull 14. The blades 56 of ice cuttingmechanisms 54 and 55 are carried by a hollow axle 62 which extendstransversely of the hull within the hull and through the upper portionsof housings 61. A pair of indexing devices 63 are connected to the axleon either side of central housing 61, and are operable for rotating theaxle through an arc of about 90 degrees for moving the ice cuttingmechanisms between their operative and retracted positions. Axle 62 isso positioned relative to housings 61, and the housings are soconfigured in cooperation with the ice cutting mechanisms, that when theice cutting mechanisms are in their retracted positions within recesses60, the mechanisms are disposed wholly above the bottom surface 16 ofhull 14. A suitable sprocket wheel (not shown, but

well known from the chain saw technology) cooperates with each chain 57coaxially of axle 62 for driving chains 57 around the circumference ofrespective blades 56. The sprockets are secured to a drive shaft 64which extends coaxially of the axle as shown in FIG. 3. The drive shaftis connected at its opposite ends to a pair of drive motors 65 mountedto the structure of bull 14 within the hull.

As shown in FIG. 3, the teeth 58 of central ice cutting mechanism 54have a greater width transversely of the vehicle 10 than do the teeth 58of the side ice cutting mechanisms. Teeth 58 are sized to cut a channel67 through ice layer 11, which channel has a width sufficiently greaterthan the outer diameter of dredgings conductor tube 28 that theconductor tube may be passed readily through this channel. Teeth 58,associated with side ice cutting mechanisms 55, are sized to cutsomewhat narrower channels 63 through ice layer 11 on either side of andparallel to channel 67, thereby to define a pair of strips 69 of icewhich are severed from ice layer 11 by operation of the ice cuttingmechanisms. The aggregate width of channels 67 and 68 is greater thanthe width of stinger assembly 52 provided in conjunction with pipelayingfacility 39. Preferably the forward end of the stinger assembly ispointed to wedge ice strips 69 apart from each other to permit the freepassage of the stinger assembly, or at least its upper end, through awidened channel 51 which has a width equal to the aggregate of the widthof channels 67 and 68.

It is apparent, therefore, that ice cutting assembly 50 provides a meansfor forming an open channel through ice sheet 11 as vehicle 10 movesover the ice above the path along which it is desired to lay pipeline45.

It will be observed from FIG. 1 that ice cutting assembly 50 is disposedwholly within the limits of the air cushion support area for vehicle 10when it is operated in its air cushion mode. As will be described below,suitable means are provided in cooperation with stinger assembly 52 forsealing skirt assembly 19 to the stinger assembly at the aft end of thevehicle. Accordingly, the front end of channel 51 is at all timesdefined at a point below hull 14 within skirt assembly 19. Thus, thegeneration of open channel 51 through ice layer 11 does not provide asource of leakage for air from chamber 20 during air cushion modeoperation of the vehicle and the vehicle is effectively supported on icelayer 11 by a cushion of air when blowers 22 are operated.

Under some conditions of ice thickness and structural characteristics,the force applied to the ice by the pressurized air within skirtassembly 19 may be effective to break the ice adequately to enablepassage of dredgings conductor tube 28., pipe continuum 44, and stingerassembly 52 through the ice. Ice cutting assembly 50 is not requiredunder such circumstances.

Stinger 52 is an elongate buoyant structure which has a width which issomewhat less than the aggregate width of ice channels 67 and 68 formedthrough ice layer 11 by operation of ice cutting mechanisms 54and 55.The stinger has an enlarged upper end 711 and varies appropriately incross-sectional configuration along its length so that when the stingerassembly is submerged and engaged with pipe continuum 44 as shown inFIG. 1, the buoyant forces developed by the submerged stinger areapplied in the desired manner to the pipe continuum. The stinger isprovided to impart support to pipe continuum 44 so that the pipecontinuum, as it passes from the vehicle into the water, is not bentabout 7 a sufficiently small radius of curvature to result in kinking orother damage to the pipe continuum. The support forces applied to thepipe continuum by the stinger also assures that the bend of thecontinuum as it engages the bottom of trench 35 is not so sharp as toproduce kinking or other damage to the pipe.

The upper portions of stinger 52 are transversely dished along thelength of the stinger to provide a guide channel 71 (see FIG. alongwhich pipe continuum 44 is passed during pipelaying operations. Theinterior of the stinger is hollow and preferably is subdivided bypartitions 72 to prevent complete flooding of the stinger in the eventthat a leak should develop through the stinger structure at somelocalized point. As shown in FIG. I, the upper end 70 of the stinger issecured, as by tether cables 73, to the underside of hull 14 within theperimeter of the space enclosed by skirt assembly 19. The connection ofthe stinger to the hull is so arranged that the pipe guide channel 71defined in the upper portions of the stinger defines an extension of thepipe supporting surfaces of passage 47 through the hull when the pipecontinuum is engaged with the stinger as shown in FIG. 1.

Stinger 52 is used during pipelaying operations whether or not thepipelaying operations are carried out on an ice layer. In the absence ofice over the intended path of the pipeline, or in the event the water isice covered but not sufficiently to interfere with floating movement ofhull 14, the hull is buoyantly supported in water 12. On the other hand,where the intended path of the pipeline is covered by a layer of icesufficiently thick to impair or prevent floating movement of hull 14,then pipelaying operations are carried out from vehicle while thevehicle is operated in its air cushion mode. FIG. 5 illustrates thatstinger 52 includes structure for maintaining an effective seal aroundspace in the vicinity of ice channel 51 during air cushion modeoperation of the vehicle.

Skirt assembly 19 includes an outer sheet 75 which is connected alongits upper margin 76 to the lower portions of plenum structure 17outboard of hull 14. Outer sheet 75 also has a lower edge 77 which,during air cushion mode operation of vehicle 10 is disposed below thebottom 16 of hull 14. The outer sheet of skirt assembly 19 preferably isheld during air cushion mode operation of the vehicle in a desiredrelation to the structure of hull 14 by a plurality of webs 78interconnected between the hull and the inner surfaces of outer sheet75. Preferably, outer sheet 75 is fabricated of a fabric impregnatedwith a suitable material such as latex or a latex-based material torender the same impervious to the flow of air through the fabric.

A yoke member 79 is connected to stinger 52 to straddle channel 71 alongwhich pipe continuum 44 passes during pipe-laying operations. A flapmember 80, preferably fabricated of much the same material as outersheet 75 of skirt assembly 19, is connected to the upper portion ofstinger 52 in the vicinity of yoke 79. Flap 80, adjacent the location atwhich the pipe continuum passes through the yoke, is carried by theexterior portion of the yoke. The clearance within yoke 79 for pipecontinuum 44 is relatively small so that a minimum leakage of air fromspace 20 through the yoke is afforded in those few operationalsituations where yoke 79 is not fully submerged in water 12. Flap 80 isdisposed to extend into skirt assembly 19 along the inner surfaces ofouter sheet 75 for a selected distance above the lower edge 77 of theskirt assembly during air cushion mode operation of the vehicle. Theincreased air pressure within space 20 biases the flap into registrationwith the inner surfaces of skirt outer sheet 75. Preferably, the loweredge 77 of the skirt assembly is disposed in spaced relation to theproximate portions of stinger 52 during pipelaying operations while thevehicle is operated in its air cushion mode. This spacing between thestinger and the lower portions of the skirt assembly allows the stingerto move relative to the vehicle hull about the connection provided bytether cables 73, and such relative motion is accommodated by flapsliding within skirt assembly 19. Preferably the extent of flap 80transversely of the length of stinger 52 is a small amount greater thanthe width of channel 51 within which stinger 52 is disposed during aircushion mode operation of the vehicle. It will be apparent, therefore,that when vehicle 10 is operated in its air cushion mode duringpipelaying operations, flap 80 forms an effective seal across the upperportions of channel 51 below skirt assembly 19. In this manner, undueleakage from space 20 to the atmosphere around vehicle 10 is preventedso that blowers 22 may be operated to maintain an effective air cushionwithin space 20 for the support of hall 14.

Preferably the spacing of skirt assembly partitions 78 at the aft end ofhall 14 is selected so that the partitions closest to the centerline ofthe hull are spaced from each other a distance greater than the extentof flap 80 transversely of stinger 52.

FIG. 4 illustrates the manner in which vehicle 10 preferably is movedacross the ice sheet during dredging and pipelaying operations. In FIG.10, only the dredging facility is illustrated, it being understood thatthe pipelaying facility, which has been eliminated from FIG. 4 for thepurposes of simplicity of illustration, is also present. On the otherhand, the principles illustrated in FIG. 4 are applicable to movement ofdredging and pipelaying platforms provided separately from each otheracross an ice sheet for performance of dredging or pipelayingoperations, respectively. As shown in FIG. 4, a winch 81 is mounted tohull deck 15 adjacent each corner of the hull; the hull preferably is ofrectangular planform configuration. Preferably each winch is disposed sothat a mooring cable 82 wound upon the winch drum is conveniently ledaway from the hull along a line substantially parallel to a diagonalline across the hull. Each mooring cable 82 extends to an anchor point83 affixed in ice sheet 11. The anchor points may be provided by anysuitable means, as by pilings frozen into holes formed in ice sheet 11.Two rows of anchor points 83 are provided parallel to the intended pathof vehicle 10 across the ice sheet and on opposite sides equidistantlyfrom the desired path of the vehicle.

Anchor points 83 are installed in ice sheet 11 in advance of themovement of vehicle 10 across the ice sheet. The spacing of the anchorpoints in each parallel line of anchor points is such that the vehiclemay be moved along its intended path of movement by selective reeling inand paying out of mooring cables 82 from winches 81. For example,mooring cables associated with the winches at the forward corners ofvehicle 10 are reeled in while the cables extending away from thewinches at the aft corners of the vehicle are paid out. In this manner,the vehicle is caused to move along its intended path of motion, andduring such movement the platform is maintained by the cables in thedesired position on the ice sheet. When the vehicle is advanced to apoint at which the forward cables extend substantially transversely ofthe vehicle to their anchor points (such as anchor points 83), first oneand then the other of the forward mooring cables are transferred to thenext pair of anchor points more forwardly of the vehicle along its pathof motion. The vehicle is moved still further along its path untilanchor points 83' are located substantially transversely from, butslightly behind vehicle 10, at which time the aft mooring cables areremoved from anchor points 83 and connected to anchor points 83'. Inthis manner, the vehicle is moved on an air cushion support across theice sheet while being maintained at all times securely in the desiredposition over the intended path of pipeline 45. Anchor points 83 leftbehind vehicle 10 as it moves across the ice sheet may be recovered asdesired.

This invention also provides an air cushion dredging vehicle 90. Asshown in FIG. 6, vehicle 90 includes a buoyant hull 14 which carries adredging facility 26. Dredging vehicle 90 is arranged for operationeither in a buoyant state, in which hull 14 floats upon water 12, or inan air cushion state, in which the hull is supported on a cushion or airgenerated within space bounded by skirt assembly 19 disposedcircumferentially of hull 14. Air is supplied at superatmosphericpressure to space 20 via skirt assembly 19 through an air distributionplenum structure 17 in response to operation of blowers 22, for example.Dredging facility 26' includes a dredgings conductor tube 91, the upperend of which is connected to the suction port of pump 30 and the lowerend of which is connected to a dredging head 92. The dredging headpreferably is of the rotary type operated in response to the passage ofwater upwardly through the conductor tube by operation of pump 30.Conductor tube 91 differs from conductor tube 28, illustrated in FIG. 1,by the inclusion in its length of a flexible section 93 which preferablyis disposed sufficiently below hull 14 as to be positioned below thebottom of ice sheet 11 when vehicle 90 is operated in its air cushionmode over ice. Whereas the dredging facility provided in vehicle 10 isadapted primarily for forming trench into which pipeline 45 may be laid,dredging facility 26' of vehicle 90 is adapted for general purposedredging on a year-round basis in waters susceptible of being covered byan ice sheet of appreciable thickness. The provision of rotary dredginghead 92 and flexible section 93 in dredgings tube 91 adapt dredgingfacility 26' to general purpose dredging operations. To adapt vehicle 90for dredging operation through ice sheet 11, a single ice cuttingmechanism 54 according to the foregoing description is carried byvehicle 90 for forming through ice sheet 11 a single channel 94 ofsufficient width that conductor tube 91 and dredging head 92 may bepassed through the channel.

As shown in FIG. 6, conductor tube 91 above flexible portion 93 passesthrough a well 29 formed through hull 14. A suitable seal 36 is providedbetween the hull and the conductor tube within the well for the purposesdescribed above. To adapt the vehicle for dredging an area in bedformation 13 which is wider than ice channel 94, a collar 95 is securedto conductor tube 91 below flexible portion 93. A plurality of cables 96are connected between collar 95 and a plurality of winches 97 mounted tohull 14. Preferably winches 97 are dis posed in recesses 98 formed inthe flat bottom 16 of hull 14 adjacent conductor tube well 29.Preferably four winches 97 are provided in vehicle 90, and are lo catedfore and aft, port and starboard, respectively,

from the position at which the conductor tube passes through the bottomsurface of the hull. Selective operation of winches 97 to reel in or payout cables 96 produces movement of dredging; head 92 forward,rearwardly, or in either direction laterally relative to hull 14. Suchmovement of the lower portion of dredging tube 91 is accommodated by theflexible portion of the tube below ice sheet 11.

When the submerged location to be dredged is free of an ice cover or iscovered by an ice layer of insufficient thickness to significantlyimpair floating movement of hull 14, dredging operations are carried outfrom vehicle while the hull is buoyantly supported in the water. On theother hand, if the location to be dredged is covered by' an ice sheet ofsufficient thickness to impair or prevent floating movement of the hull,then vehicle 90 is operated in its air cushion mode and dredgingoperations are carried out at the desired submerged location through icechannel 94 provided in the ice sheet by operation of ice cuttingmechanism 54. As shown in FIG. 6, ice cutting mechanism 54 preferably ismovable between the operative position shown in FIG. 6 and a retractedposition in which the mechanism is disposed in a recess 60, preferablyprovided in the manner described above concerning vehicle 10.

The formation of channel 51, for example, through ice layer 11 duringpipelaying operations from vehicle 10, for example, provides adiscontinuity in the ice. Wind, wave or tidal forces acting on the icemay tend to close channel 51 around stinger 52, for example, or tofurther open the channel more than may be safe relative to vehicle 10.Accordingly, preferred practice of the pipelaying and dredgingoperations through ice according to this invention includes theinstallation across the ice channel behind the vehicle, such as vehicle10, of means for maintaining the channel at the desired width.Preferably the channel maintaining means are provided in the form ofsteel bars 100 bent to resemble enlarged staples in that the endportions of the bar are bent at right angles, parallel to each other,relative to the straight central portions of the bar. The centralportions of the bars have a length. which exceeds the width of thechannel, as initially formed, by a desired amount, and the end portionsof the bars have lengths preferably at least equal to the thickness ofice layer 11. The bars are installed across the channel behind thevehicle as it moves along its intended path at appropriate intervalsalong the length of the channel. The installation of the bars across thechannel may be achieved by drilling holes through the ice at alignedlocations along the channel and on opposite sides of the channel. Theholes preferably are sized to snugly receive the end portions of thebars. The bars are then installed by inserting their end portions intocorresponding ones of the aligned holes. When so installed, the bars actto inhibit opening or closing of the channel.

This invention has been described above with reference to certainpresently preferred structural arrangements and operational sequences.Those skilled in the art to which this invention pertains will readilyrealize that the structures illustrated in the accompanying drawings, aswell as the procedures and operations described above, may be altered ormodified without departing from the scope of this invention.Accordingly, the foregoing description should not be considered aslimiting this invention.

What is claimed is:

l. A method for dredging in water susceptible to being covered by alayer of ice comprising the steps of l. locating a dredging facility ona buoyant platform adapted to be supported on a cushion of air,

2. floating the platform in the water and performing dredging operationsfrom the platform during periods when the water is not covered by icesufficient to restrict floating movement of the platform, and

3. during periods when the water is covered by ice sufficient torestrict floating movement of the platform, forming an opening throughice below the platform, supporting the platform on a cushion of air, andperforming dredging operations from the platform through the ice.

2. Apparatus useful for dredging the bed of a body of water covered by alayer of ice, comprising 1. a buoyant platform,

2. a dredging facility carried by the platform including a dredging headoperable when engaged with said bed for removing material therefrom, anda dredgings conductor tube connectable between the dredging head and theplatform for conducing dredged material from the head to the platformand adapted to be lowered from the platform for engagement of thedredging head with the bed,

3. means carried by the platform for forming a channel through a layerof ice below the platform for passage of the conductor tubetherethrough,

4. means carried by the platform and operable for supporting theplatform on a cushion of air over the channel.

3. Apparatus for dredging the bed of a body of water covered by a layerof ice comprising a buoyant platform,

means carried by the platform and operable for supporting the platformon a cushion of air,

a dredging facility carried by the platform and including a dredginghead engageable with the bed for dredging the same and a conductor tubeconnectible between the dredging head and the platform 12 for conductingdredged material from the head to the platform, and

means carried by the platform operable for forming through an ice layerbelow the platform a channel through which the conductor tube isextendible, the means for supporting the platform on a cushion of airincluding a skirt assembly mounted to the platform for enclosing a spacebelow the platform, the means for forming the channel being operablethrough the space.

4. Apparatus useful for dredging the bed of a body of water covered by alayer of ice, comprising 1. a buoyant platform,

2. means carried by the platform and operable for supporting theplatform on a cushion of air, said means including a skirt assemblymounted to the platform for enclosing a space below the platform andmeans for supplying air at superatmospheric pressure to the space,

3. a dredging facility carried by the platform includ ing a dredginghead operable when engaged with said bed for removing materialtherefrom, and a dredging conductor tube connectible between thedredging head and the platform for conducting dredging material from thehead to the platform and adapted to be lowered from the platform forengagement of the dredging head with the bed, and

4. means carried by the platform operable within said space for forminga channel through a layer of ice below the platform for passage of theconductor tube therethrough,

5. Apparatus according to claim 4 wherein the channel forming means ismovable between a retracted position substantially within the platformand an operative position in which it extends into the space intooperative engagement with ice below the platform.

6. Apparatus according to claim 5 wherein the channel forming meanscomprises means for mechanically cutting through the ice.

1. A method for dredging in water susceptible to being covered by alayer of ice comprising the steps of
 1. locating a dredging facility ona buoyant platform adapted to be supported on a cushion of air, 2.floating the platform in the water and performing dredging operationsfrom the platform during periods when the water is not covered by icesufficient to restrict floating movement of the platform, and
 3. duringperiods when the water is covered by ice sufficient to restrict floatingmovement of the platform, forming an opening through ice below theplatform, supporting the platform on a cushion of air, and performingdredging operations from the platform through the ice.
 2. a dredgingfacility carried by the platform including a dredging head operable whenengaged with said bed for removing material therefrom, and a dredgingsconductor tube connectable between the dredging head and the platformfor conducing dredged material from the head to the platform and adaptedto be lowered from the platform for engagement of the dredging head withthe bed,
 2. floating the platform in the water and performing dredgingoperations from the platform during periods when the water is notcovered by ice sufficient to restrict floating movement of the platform,and
 2. Apparatus useful for dredging the bed of a body of water coveredby a layer of ice, comprising
 2. means carried by the platform andoperable for supporting the platform on a cushion of air, said meansincluding a skirt assembly mounted to the platform for enclosing a spacebelow the platform and means for supplying air at superatmosphericpressure to the space,
 3. means carried by the platform for forming achannel through a layer of ice below the platform for passage of theconductor tube therethrough,
 3. during periods when the water is coveredby ice sufficient to restrict floating movement of the platform, formingan opening through ice below the platform, supporting the platform on acushion of air, and performing dredging operations from the platformthrough the ice.
 3. Apparatus for dredging the bed of a body of watercovered by a layer of ice comprising a buoyant platform, means carriedby the platform and operable for supporting the platform on a cushion ofair, a dredging facility carried by the platform and including adredging head engageable with the bed for dredging the same and aconductor tube connectible between the dredging head and the platformfor conducting dredged material from the head to the platform, and meanscarried by the platform operable for forming through an ice layer belowthe platform a channel through which the conductor tube is extendible,the means for supporting the platform on a cushion of air including askirt assembly mounted to the platform for enclosing a space below theplatform, the means for forming the channel being operable through thespace.
 3. a dredging facility carried by the platform including adredging head operable when engaged with said bed for removing materialtherefrom, and a dredging conductor tube connectible between thedredging head and the platform for conducting dredging material from thehead to the platform and adapted to be lowered from the platform forengagement of the dredging head with the bed, and
 4. means carried bythe platform and operable for supporting the platform on a cushion ofair over the channel.
 4. means carried by the plaTform operable withinsaid space for forming a channel through a layer of ice below theplatform for passage of the conductor tube therethrough.
 4. Apparatususeful for dredging the bed of a body of water covered by a layer ofice, comprising
 5. Apparatus according to claim 4 wherein the channelforming means is movable between a retracted position substantiallywithin the platform and an operative position in which it extends intothe space into operative engagement with ice below the platform. 6.Apparatus according to claim 5 wherein the channel forming meanscomprises means for mechanically cutting through the ice.